1. Field of the Invention
An aspect of the present invention relates to a sub-carrier diversity method on a multi-band (MB)-orthogonal-frequency-division-multiplexing (OFDM) symbol, and more particularly, to a sub-carrier diversity method on an MB-OFDM symbol by which a time domain spreading (TDS) is set with respect to sub-carriers according to a frequency hopping pattern and positions of the sub-carriers are shifted in a predetermined unit within each time domain so as to allow the sub-carriers to diverge in an MB-OFDM system in which an identical frequency band of an identical symbol is repeated.
2. Description of the Related Art
In general, OFDM systems transform symbols input in series into parallel symbols having predetermined sizes, multiplex the parallel symbols into orthogonal different sub-carrier frequencies, and transmit the orthogonal different sub-carrier frequencies.
In an MB-OFDM method, a plurality of frequency bands of an OFDM symbol in the unit of symbol hop in order to transmit a signal. For example, the MB-OFDM method is a modulation technology used in a specific wireless communication system such as an ultra wide band (UWB) system. OFDM modulation technology and frequency hopping technology are combined into MB-OFDM modulation technology.
An MB-OFDM system divides a predetermined frequency band into a plurality of sub-bands. The MB-OFDM system can transmit data (a symbol) using the plurality of sub-bands so as to transmit or receive a large amount of data per unit time. A UWB system selects one of the pluralities of sub-bands and uses the selected sub-band according to set regulations so as to improve security of data.
[Table 1] below shows a method of transmitting payloads according to a transmission rate in the MB-OFDM system.
TABLE 1TransmissionModulationEncodingSpreadingRateMethodRateConjugateTSFGain53.3QPSK1/3∘2480QPSK1/2∘24106.67QPSK1/3x22160QPSK1/2x22200QPSK5/8x22320DCM1/2x11400DCM5/8x11480DCM3/4x11
In a case where the transmission rate is between 53.3 Mbps and 200 Mbps, the MB-OFDM system uses a quadrature phase shift keying (QPSK) method. In a case where the transmission rate is between 320 Mbps and 480 Mbps, the MB-OFDM system uses a dual carrier modulation (DCM) method.
In a case where the transmission rate is between 53.3 Mbps and 80 Mbps, the MB-OFDM system transmits a conjugate symbol. Thus, the spreading gain is “4.” In other words, in a case where the transmission rate is between 53.3 Mbps and 80 Mbps, a time spreading factor (TSF) is “2.” Thus, one symbol is transmitted four times, including conjugate symbols.
[Table 2] below shows an example of transmitting a symbol in an MB-OFDM system having a transmission rate between 53.3 Mbps and 80 Mbps.
TABLE 2DataMapping DataD0C0D1C1. . .. . .D49C49D49*C50. . .. . .D1*C98D0*C99
Referring to Table 2, one piece of data is transmitted two times, including conjugate data. In other words, a transmitter transmits data D0 through D49 together with conjugate data D0* through D49*. Also, if the QPSK method is used, the transmitter divides one piece of data into real and imaginary components and transmits the real and imaginary components.
In a case where a data transmission rate is between 53.3 Mbps and 200 Mbps in an MB-OFDM-based UWB system as described above, a value of a TDS is “2” as shown in FIG. 1. Thus, the MB-OFDM-based UWB system transmits a frequency band of a sub-carrier including a symbol and a conjugate symbol only two times. As a result, in a case where errors occur in transmitted frequency bands Band1 and Band2, a bit error rate (BER) is increased.